Dorsal-ventral patterning in the fruit fly Drosophila melanogaster is accomplished through a complex series of signal transduction pathways. Central to these pathways are a series of serine protease-like molecules that are thought to transmit a localized, extracellular, ventralizing signal to prospective ventral cells. One of these protease-like molecules is the Easter protein. The Easter protein is a member of the Trypsin family of serine proteases. Genetic studies suggest that Easter is indeed a serine protease, but biochemical evidence directly demonstrating a proteolytic activity is still lacking. Genetic studies have also identified a large number of mutations in Easter that cause loss of Easter function or gain of Easter function. In hopes of understanding the biochemical activity of Easter and determine the molecular basis of the phenotypes seen in these mutant lines, we have developed a homologous expression system for Easter using Drosophila Schneider cells (S2 cells). Currently, we are just beginning the characterization of the Easter protein and are in the process of testing Easter proteolytic activity. In the near future, we plan to express the mutant forms of Easter isolated genetically to determine how these mutant forms of Easter cause changes in phenotype. The Computer Graphics Laboratory (CGL) will play a rucial role in the rationalization of these mutations. By mapping these mutations onto a model of the Easter protein, using the Trypsin crystal structure as a model base, we hope to gain insight into how Easter structure relates to its biological function.